1. The Field of the Invention
The present invention pertains to methods and systems using reverse osmosis for water purification and, in particular, to domestic units which are readily adaptable to treat local water in accordance with any existing long term or varying temporary condition to produce water of high purity and to a flowboard for mounting fluid purification elements and controlling flow distribution in the system.
2. The Prior Art
The availability of quality potable water is becoming an increasing national and world concern. There are many areas where local water has been subject to mismanagement and the quality thereof has gone down dramatically to the point that it may be dangerous for human consumption. There also are areas, such as west Texas, where natural fluorides occur at such high levels that the water will adversely affect the teeth of young children leaving them with discolored teeth for life. There are still other areas, such as the Dakotas, where the water naturally contains high levels of sulfides making the water practically undrinkable to many. Further there are periodic local and/or regional occurrences, such as the weather related natural disasters of flooding and hurricanes, when local water supplies may become contaminated and at least temporarily unsafe for drinking. Theretofore, the accepted solution to these problems has been the importation of bottled water. This often is at great cost and bottled water may not always be available, such as in times of natural disasters when both water supplies and modes of transportation are damaged. Further, there is no assurance that any imported bottled water will have acceptable purity as there currently are no federal, state, or local standards in force which would have to be met by the bottlers.
Impurities in natural raw waters (surface or well water) occur in four basic different forms, namely non-ionic and undissolved impurities; ionic and dissolved impurities; gaseous impurities and biological impurities. Each of these impurities requires separate treatment techniques and equipment for their removal.
Non-ionic and undissolved impurities include, but are not limited to, turbidity, silt, mud, suspended solids, organic matter, bacteria, oil, colloidal matter and colloidal silica. A common technique to remove such impurities is filtration using a wide variety of coarse and fine filter media. Some other techniques include coagulation, sedimentation and adsorption on activated charcoal or carbon. In raw water treatment, it is customary to use a coarse filter (sand and anthracite) followed by a fine filter (cartridge), and then to treat the water with activated carbon to remove organic matter. In cases using pretreated water, such as municipal water supply, the first filtration with course sand is generally unnecessary.
Ionic and dissolved impurities include a wide variety of salts dissolved in water and dissociated to form positive ions, called cations, and negative ions, called anions. The major cations in natural raw water are calcium, magnesium, sodium, potassium, ammonium, iron and manganese. The major anions are carbonate, bicarbonate, hydroxide, chloride, sulfate, nitrate, phosphate, and silica. Both of these lists are intended to be representative and should not be considered in any way as all-inclusive. Both the positive ions and the negative ions combine in various fashions to form a large group of compounds that would dictate the treatment process for their removal. For example, calcium and magnesium form carbonates, which, in turn, cause water hardness. Water containing large amounts of carbonates must be softened to prevent fouling and clogging of equipment and other separation media.
Natural waters also include traces of different heavy metals. Several means are used to partially or completely remove such impurities. U.S. Pat. No. 5,190,659 to Wang, is a good example of water conditioning, or partial treatment, for removal of these metals. Complete removal of metal impurities requires other techniques, such as evaporation, membrane separation and ion exchange.
Gaseous impurities include a number of gases that are soluble in water. Some are found naturally in well water, such as carbon dioxide, hydrogen sulfide, and methane. Others are the result of water purification or industrial application and include such gases as ammonia, oxygen and chlorine. In these cases, aeration, oxidation, stripping or an oxidizing catalyst, such as manganese green sand zeolite, is usually a practical means for removing the dissolved gases.
Biological impurities include all types of microorganisms, bacteria, viruses, and pathogen. Several disinfection methods are available for treating this type of impurity including boiling (limited to small volumes), chlorination, ozonation and ultraviolet radiation.
In most cases, all of these four forms of impurities coexist simultaneously and in differing amounts and their relative proportions can vary, even seasonally. No single treatment or technique is adequate for or capable of removing all impurities in one step. Multiple related or interdependent processes are normally required to rid water from such impurities.
Generally these processes must be constantly monitored to assure each form of impurity is being properly treated and removed.
Membrane separation is an economic means for removing dissolved solids from water that contains low concentration of these solids. Development of membranes for water purification is a consequence of a naturally occurring phenomenon called Osmosis. Osmosis is a spontaneous flow of pure water from a weaker to a stronger solution through a semi-permeable membrane, such as live plant cell walls or a synthetic cellulose acetate film. Solvent (water in this case) ceases to flow when the generated differential head (water column height) across the membrane equalizes the force generated by the chemical potential difference (molecular concentration) across this membrane. The net force exerted by the flow of water across the membrane is called osmotic pressure. Reverse osmosis (RO) is a pressure driven membrane operation and in fact is the reverse process of osmosis. Under pressure exceeding the osmotic pressure of the solution, water or solvent permeates through a semi-permeable membrane from a more concentrated stronger solution to a less concentrated solution. Rejection of dissolved solids could reach 98% depending on the applied force and the original concentration of the solution. For example, water containing 500 PPM of dissolved solids could be purified with reverse osmosis to about 10 PPM.
Reverse osmosis (RO) synthetic membrane materials can be formed in shape of hollow fine fibers, tubular, plate and frame or spiral-wound. The spiral-wound membrane is an envelope of two flat films enclosing a flexible porous substrate to facilitate permeate flow and is sealed on three of its edges. The open edge is connected and rolled up onto a perforated permeate tube forming a spiral. These envelope-like membranes are separated from one another with a corrugated or mesh spacer to allow open feed flow. Under pressure, water permeates through the film and collects in the concentric perforated tube while the concentrated solution continues flowing to an outlet opposite to the feed inlet. Most of the reverse osmosis systems for commercial and residential water purification system use the later type of membranes in the form of an elongated cylinder housed in a disposable cartridge. Each cartridge has a feed water inlet at one end and permeate (purified water) and reject (high concentrated brine) outlets at the other end. An example of such a cartridge is described in U.S. Pat. Nos. 3,504,796 and 4,842,736, both to Bray.
Membranes are very susceptible to molecular, biological or sedimentary fouling that can easily reduce their useful life. To maintain longevity of membranes (normally three years), water pretreatment is usually required. This pretreatment includes filtration, organic removal, softening and disinfection. For domestic systems, usually the first two pre-treatment steps are provided. Membranes are highly effective in retaining bacteria and pathogen. In most cases, post-treatment is not required. Unfortunately, biological contamination of water purified with reverse osmosis is common. Most membranes are sensitive to oxidizers, such as chlorine and other similar disinfectant substances. These disinfectants must be removed from water prior to treatment to avoid premature membrane failure. As a result, water enters the membrane with no disinfectant residual that could inhibit microbiological growth on these membranes and further contaminating water during handling and storage.
Furthermore, reverse osmosis membranes are relatively low capacity units. For example, a conventional 12" reverse osmosis cartridge that is normally used for household can only produce between 2-8 gallons per day at municipal water supply pressure. To make up for the slow water production, a storage tank is always provided to collect and store the slow flow of permeate to meet instantaneous demand. For under the counter units, a bladder tank (3-5 gallons) is provided. In these tanks, water is stored in a bladder. The annulus between the bladder and the tank wall is permanently pressurized with air up to 15 psi to make it possible to dispense water on demand. A five-gallon bladder tank is aesthetically unacceptable option for counter-top applications.
The capacity of these membranes is highly dependent on the water supply pressure. An increase in line pressure will proportionally increase the amount of produced water. For this reason, large commercial and industrial systems are provided with booster pumps. Small household systems operate under the hydraulic pressure of the municipal supply line with a minimum pressure of 35 psi. Operating a reverse osmosis under this low pressure and, against back pressure of 15-psi in the storage tank, can significantly reduce the capacity of the membrane.
Household reverse osmosis systems consist of more than one module for water purification. These systems also require adequate means to store purified water. Commonly known systems are bulky and require an experienced technician to install and maintain. Due to their size, they are normally mounted under kitchen sink. Such systems are impractical from an anesthetic standpoint to place on a counter.
Examples of known approaches to water treatment systems and apparatus incorporating reverse osmosis are described in the following patents.
U.S. Pat. No. 5,045,197 to Burrows describes a conventional under sink, reverse osmosis water purification system. It comprises of a unitary wall mounted manifold to mount a reverse osmosis cartridge and associated pre and post filtration elements. The system comprises an under-sink water storage vessel equipped with a bladder
U.S. Pat. No. 4,744,895 to Gales et al relates to a counter top reverse osmosis water purifier connected to the end of a household water faucet, using a detachable diverter valve. The purifier comprises a housing that encompasses reverse osmosis element and filtration modules in the lower section of the housing. An atmospheric water reservoir occupies the upper section of the housing. A water conductivity device and battery operated pump are also installed in the housing to for controlling and dispensing purified water. Several pipes and fittings protrude through the atmospheric reservoir making its sanitization difficult. The purifier provides no provisions to prevent stored water contamination, which would place its effectiveness for household water purification into question.
U.S. Pat. No. 5,078,876 to Whittier et al describes a household, concentric multistage potable water purification system. The system is counter top mounted. It consists of a detachable cylindrical vessel mounted on a base and houses one or more replaceable, shell formed concentric purification modules. These modules could include filtration media, reverse osmosis element and an optional Ultraviolet lamp spaced radially in the vessel. Purified water is discharged from the purifier and to be collected by the user (no reservoir is shown). The said patent provides compact multistage, single unit design, but without adequate provisions for water quality measurement and control, handling of exhausted elements that could be biologically contaminated, system sanitization, and adequate means for water storage.
U.S. Pat. No. 5,096,574 to Birdsong et al discloses a counter top reverse osmosis system consisting of a cabinet containing three cavities which respectively receive cartridges housing a sediment filter, reverse osmosis filter and a final stage impurity filter. Permeate from the reverse osmosis is fed to a bladder within a storage tank. Concentrate from the reverse osmosis filter is used within the storage tank as squeeze water for the bladder. Complicated systems for constant delivery of consumer water usage, operating under wide range of pressure and monitor flow are provided. The disclosure does not provide means to sanitize water being stored in the tank bladder.
U.S. Pat. No. 5,358,635 to Frank et al describes a reverse osmosis water processing and storage system. It comprises an external self supported vertical pressure vessel that receives at its open top a bladder forming a variable volume between the bladder and the external tank. A second internal tank is received and secured in the bladder, forming a second variable volume between the second tank and the bladder. A concentric pre-filter and a reverse osmosis membrane assembly is received and secured in the second tank. Raw water enters the second tank and flows through the pre-filter and then through the reverse osmosis membrane. The purified water thereafter flows to the second variable storage where it can be dispensed for usage under the accumulated pressure in the external tank. Due to vessel elevation and flexible piping layout, it is safer to install this system in a confined space such as under a sink.
U.S. Pat. No. 5,445,729 to Monroe et al describes a counter top reverse osmosis system which operates under low back pressure. The unit consists of housing suitable for counter top mounting and encompasses two compartments. The first compartment contains water treatment elements including, pre and post filters, ultraviolet light means, and a reverse osmosis membrane. The second compartment contains bladder type water storage and a pump to withdraw water for dispensing. The system is compact and includes an array of water treatment elements, but seems to undermine maintenance simplicity. Furthermore, the system relies on electrical power to dispense water. Failure of the power source could render the system inoperable.
It is an object of the present invention to provide a method and system for point-of-use, counter top, reverse osmosis water purification. The subject method and system are primarily intended for residential and lower volume commercial markets by improving the aesthetic quality of water and controlling, or substantially eliminating, any and all ionic and/or microbiological contamination naturally occurring in the raw water. Each unit has a flowboard, which can be specially configured for the local water supply with replaceable cartridges for filtration, adsorption, reverse osmosis, ion exchange, and biological disinfection. The flowboard also allows configuration for providing additives to the water during treatment. The unit can be provided with digital instrumentation. Operational safety and ease of maintenance are key features of the present invention.